DNA STRUCTURE & REPLICATION
DNA STRUCTURE & REPLICATION
A MODEL OF DNA
• In 1953, two scientists named Watson & Crick
built a model of DNA that demonstrates its
exact structure and function.
• They called this model a double helix, which
is also often referred to as a twisted ladder.
• A woman named Rosalind Franklin is actually
responsible for performing an x-ray on a
chromosome. This initiated Watson & Crick’s
double-helix design.
DNA: THE THREAD OF LIFE
• DNA (also known as deoxyribonucleic acid) is a long molecule that makes up our chromosomes & is located inside the nucleus of a cell.
• DNA is known as the thread of life because it contains all of the information to direct our body’s functions in order to sustain life.
CHROMOSOMES
• Chromosomes are made of
DNA
• Segments of DNA in specific
patterns are called genes
• Chromosomes are not always
visible. They are uncoiled and
as loose strands called
chromatin
• When it’s time for the cell to reproduce,
they condense and wrap up very tightly
to form a chromosome
THE ROLE OF DNA
• The DNA that makes up genes must be capable of storing,
copying, and transmitting the genetic information in a cell.
• The 3 functions of DNA are:
• 1. Storing Information
• 2. Copying information
• Before a cell divides, it must make a complex copy
of every one of its genes
• 3. Transmitting information
• DNA must be carefully sorted and passed along
during cell division so that it is transmitted from one
generation to the next.
THE STRUCTURE OF DNA
• DNA is a long molecule made
up of monomers called
nucleotides
• Nucleotides are made of three
parts
1.5-carbon sugar
(deoxyribose)
2.Phosphate group
3.Nitrogenous base
NITROGENOUS BASES
• There are four kinds of nitrogenous bases.
• They are divided into two types- purines and pyrimidines.
• Purines: Adenine and
Guanine
• Pyrimidines: Cytosine
and Thymine
BASE PAIRING
• Because of the size and shape of the
nitrogen bases, there are certain ones
that always pair together.
• Adenine & Thymine (A-T)
• Cytosine & Guanine (C-G)
• Chargaff’s Rule states that the pyrimidine
& purine bases have a 1:1 ratio
• The amount of guanine is equal to cytosine
• The amount of adenine is equal to thymine
• These nitrogenous bases form the
rungs/steps of the DNA molecule
HYDROGEN BONDS
•Weak hydrogen bonds form between the nitrogen
bases to hold the two strands of DNA together
• Adenine and Thymine are held together by a double hydrogen bond
• Cytosine and Guanine are held together by a triple hydrogen bond
DNA’S ANTIPARALLEL STRUCTURE
• The strands of DNA are
antiparallel
• the two strands run in
opposite directions
• All DNA strands are
read from the 5' to the
3' end
• the 5' end ends with a
phosphate group
• the 3' end ends with a
sugar molecule
DNA REPLICATION
• DNA Replication is the copying process by which a cell duplicates its DNA
•Occurs during S phase of the cell cycle in the nucleus
• In order to do this, the enzyme DNA helicase moves along a molecule of DNA and breaks the weak hydrogen bonds between the nitrogen bases.
DNA REPLICATION
When they do this they
“unzip” the ladder,
which comes apart
and the two sides of
the ladder separate.
• DNA polymerase comes along
afterward and carries the individual
nucleotides to the site of replication
• builds the daughter strand by matching
new nucleotides to their complementary
bases on the parent strand
• DNA polymerase elongates the DNA
strand in the 5’-3’ direction only
• Because of this, DNA polymerase has to
work backwards on one strand
• DNA replication proceeds until each
chromosome is completely copied.
DNA REPLICATION
DNA REPLICATION
DNA REPLICATION
• Each new strand of DNA now has half
of the old strand that came apart
and half of a new strand that was just
created.
• At the end of replication, there are 2
new identical strands of DNA
• 1 side is from the original DNA
strand (parent/template)
• The other side is the newly formed
strand that was “copied”
(daughter)
DNA REPLICATION
DNA REPLICATION